MTMR9 is an adapter protein for catalytically active myotubularin-related phosphatases that regulates phosphoinositide metabolism and cellular trafficking. As an enzymatically inactive member of the myotubularin family, MTMR9 functions by forming heteromeric complexes with active phosphatases: it increases MTMR6 phosphatase activity up to 6-fold toward phosphatidylinositol 3,5-bisphosphate 1, enhances MTMR8 catalytic activity toward phosphatidylinositol 3-phosphate 2, and positively regulates MTMR7 activity 3. MTMR9 stabilizes both itself and its binding partners through complex formation, preventing their degradation 12. Subcellularly, MTMR9 localizes to the Golgi apparatus and intermediate compartment, recruiting active phosphatases to these locations and regulating ER-to-Golgi trafficking and secretion of cargo proteins like WNT3A 4. Functionally, MTMR9 negatively regulates autophagy through MTMR8 association 2 and protects against DNA damage-induced apoptosis via MTMR6 1. Clinically, mutations in MTMR9 are associated with nonsyndromic intellectual disability in Chinese populations 5, while the rs2293855 polymorphism associates with increased atherogenic risk in children 6. MTMR9 variants have been identified as candidate disease genes in diverse genetic disorders 78.